1. Field of the Invention
The present invention relates to a dielectric material for semiconductor devices and, more particularly, to a dielectric that utilizes ion implantation of fluorine into oxide films.
2. Description of the Related Art
For 0.25 .mu.m ULSI technology and smaller, materials having a dielectric constant K of less than 3 are preferred. Such low-K dielectric materials reduce the parasitic capacitance between adjacent metal lines that can give rise to cross-talking. In addition, low-K dielectric materials reduce the signal time delay .tau., where .tau.=RC.about.RK, K being the dielectric constant and R being the resistance.
A variety of low-K materials have been studied, ranging from fluorosilicate glass (FSG) to more exotic materials such as amorphous carbon films. Because of the ubiquity of oxide in present processing technology, it would be most convenient if the goal of a low-K dielectric can be achieved using SiO.sub.2.
FSG films can be successfully integrated into existing ULSI processes only with an effective dielectric constant of about 3.5 or greater. FSG films are conventionally formed utilizing plasma enhanced chemical vapor deposition (PECVD) or high density plasma chemical vapor deposition (HDPCVD) techniques, utilizing SiF.sub.4 gas as the fluorine source. Increasing the ambient SiF.sub.4 concentration decreases the dielectric constant of the FSG produced. Unfortunately however, the relatively low energies present during CVD processes (1-2 eV) result in the deposited F being weakly incorporated into the SiO.sub.2. This incomplete bonding of fluorine within oxide poses several serious problems.
First, upon heating, the weakly bound and volatile F may evolve as a gas. The fluorine gas may reach interconnect metallization layers adjacent to the FSG, causing delamination at the metal/FSG interface.
Second, incomplete incorporation of F into the SiO.sub.2, combined with subsequent outgassing as described above, may produce gaps or vacancies within the SiO.sub.2 lattice. When FSG films possessing such defects are exposed to water, water may penetrate into the film. Water can be absorbed by the FSG in two forms: as Si(OH) or as H(OH). The presence of either Si(OH) or H(OH) in the FSG is problematic.
Si(OH) will evolve as water at temperatures above 600.degree. C. H(OH) will evolve as water at temperatures above 200.degree. C. The water so generated can disrupt subsequent high temperature processing steps.
In addition, the presence of Si(OH) in the FSG film increases its dielectric constant, negating the benefit conferred by higher concentrations of fluorine. Furthermore, the presence of H(OH) in the FSG film can lead to corrosion of adjacent metal lines and via poisoning.
Therefore, it is desirable to prepare and utilize a FSG film containing high concentrations of fluorine that are stably incorporated within the oxide.